This invention relates to a wear-resisting protective layer of a magnetic field modulation type magneto-optical disk, and more particularly to a structure of magneto-optical disk which is suitable for obviating the collision, adhesion or the like between a rotating disk and a magnetic head having an air floating function.
The conventional magnetic field modulation type optical disks have been said to be capable of overwrite recording if a magneto-optical film having an easy axis of magnetization in the perpendicular direction is formed on a recording surface of a disk and the direction of magnetization in a region irradiated with a laser beam is controlled easily and at high speed by applying a magnetic field thereto, as discussed in Japanese Journal of Applied Physics, Vol. 26 (1987), pp. 231-235.
In the example mentioned above, however, a modulated magnetic field has been applied by a field generating electromagnetic coil located at a distance of 0.1 to 0.5 mm upward from the disk. With such an arrangement, the recording frequency has been less than 0.5 MHz and it has been impossible to record signals at a high frequency on the order of several megahertz or above.
In view of the above, an overwriting method based on the so-called contact start stop system (hereinafter referred to "CSS system") has been developed in which an optical head and a magnetic head are opposed to each other, with a magnetic field modulation type disk interposed therebetween, and in which an air floating magnetic head is used as the magnetic head. Recording of signals is carried out by irradiating the magnetic field modulation type recording film with a high-output laser beam from substrate side by use of the optical head, and, at the same time, applying a modulated magnetic filed, with inversion of polarity according to the signals by the magnetic head, to the disk from the recording film side. By this method, old signals can be overwritten with new signals.
However, in carrying out the CSS system, there has occurred the problem that, as shown in FIG. 2, the disk surface and the magnetic head 8 are liable to undergo wear, marring or the like due to contact wear while the disk is not rotated or when the rotation is started or stopped. In order to prevent such troubles, a method of making the disk surface rougher than the surface of the magnetic head has been proposed. Roughening the surface is carried out by, for example, grinding the surface into a rough state. In this case, the magnetic disks are subjected to grinding, one by one, after the formation of the protective layer. The grinding operation is troublesome and leads to the fatal problem of generation of dust. In addition, a mistake in the grinding operation or the like would damage the recording film beneath the protective layer. To obviate the difficulties, a method has been proposed in which an uneven surface pattern is preliminarily imparted to the surface of a transparent substrate by the above-mentioned method, and the substrate with the surface pattern is used as a mother mold for replication of the pattern by use of a UV light curable resin or the like. In this case, the rough pattern on the surface of the mother mold is a concave pattern, so that the rough pattern formed on the surface of the protective layer of the disk through replication is a convex pattern, as shown in FIG. 3.
Thus, the rough patterns of the disk surface obtained by the prior art would sometimes be irregular convex patterns, as shown in FIG. 3. In such a situation, the magnetic head would collide against the protrusions of the rough pattern, thereby being easily worn or marred or resulting in the generation of dust.
As a countermeasure against these problems, control of the depth of surface roughness of the mother mold to obtain a uniform depth of roughness has been proposed. This approach, however, has the problem that the manufacturing method and the processing method are difficult to carry out and the reliability thereof is low.
Furthermore, as the revolving speed of the disk is increased in correspondence with an increase in recording capacity, there also arises the possibility of the head being broken due to collision on the above-mentioned protrusions.
Besides, in FIGS. 2 and 3, numeral 6 denotes a substrate with a recording film, 7 denotes a protective layer, and 10 denotes a reference level.